Amazon always requires a category (Browse Node ID in Amazon terminology) for all newly created listings (i.e. the UPC/EAN that you are using to create the listing with doesn't exist on Amazon).

If you are creating an 'offer' for an existing Amazon catalog product i.e. uploading your (price+inventory+shipping) for an existing product being sold, then a category is not required.  In this case you are using the category that already exists for that ASIN so there is no need to set it. 

Setting the Amazon Category in Multi-Edit Mode

Scroll right on the Codisto XpressGrid™ view until you see the Category column in the Listing Details section.

You can set categories for individual product rows or in filtered groups or in bulk across your entire catalog with the Multi-edit function.

Amazon Category Selection Dialog

Click the  icon to expand a category to expose it's child categories. Repeat this process until you see the leaf categories, which have a radio button selection. Click a leaf category to select it, then click OK.

Amazon Category Selection Dialog showing leaf categories

Once you have added a category to your product, you can save and publish to Amazon.